1. Field of the Invention
N-acetyl-p-aminophenol, commonly known as acetaminophen, is known for a wide variety of uses, e.g., as an intermediate for pharmaceuticals and azo dyes, as a stabilizer for hydrogen peroxide, as a photographic chemical, and as a medicinal drug. Its medicinal use is the most well known, notably as a non-prescription analgesic with properties similar to aspirin. It is thus used as the active ingredient in the preparations designated paracetamol (U.K.) and Tylenol.RTM. (U.S.), and a major component in over 200 other drug formulations.
While it has several advantages over aspirin, notably a lesser tendency to promote internal bleeding, acetaminophen has unfortunately been found to exhibit hepatotoxic properties when present in large quantities in the blood stream. As a result, acetaminophen overdose has become a frequent method of suicide in recent years. Death can be avoided by the administration of an antidote, but only if the antidote is administered within 10 to 12 hours of acetaminophen ingestion. Thus, it is of the utmost importance that the acetaminophen level in the blood plasma or other bodily fluids be determined within a few hours of ingestion.
Currently known assay methods include colorimetry, UV spectrophotometry, gas liquid chromatography, high pressure liquid chromatography, fluorometry, and voltametry. Colorimetry requires either the conversion of the acetaminophen to p-aminophenol, or a dye reaction, or nitration. These are time-consuming procedures, and the accuracy of this method suffers from the potential interference of similar drugs present in the sample. UV spectrophotometry, fluorometry, and voltametry similarly suffer from interference, notably from salicylate and other compounds with phenolic hydroxy groups. While such interference can somtimes be eliminated by splitting the sample into two parts, acidifying one and rendering the other alkaline, the procedure is still time-consuming and labor-intensive. Gas and liquid chromatography are similarly time-consuming and labor-intensive, with the further disadvantage of requiring highly specialized instrumentation.
In addition to acetaminophen itself, various metabolized forms of the drug are normally present in the body fluid to be analyzed, further complicating the detection procedure. These metabolites include the glucuronide and the sulfate (each with the modifying moiety substituting for the hydroxyl hydrogen), as well as the cysteine and mercapturic acid conjugates (in each of which the added group is positioned at the C-3 ring position, i.e., the position adjacent to the position occupied by the hydroxyl group). A normal dosage of acetaminophen is 80%-90% metabolized in the liver to form the first two of these metabolites, with the rest going to the last two, leaving only about 2% of the acetaminophen unchanged. When an overdose is ingested, the capacity of the liver to form these metabolites is overloaded and the acetaminophen (through its unstable intermediate, N-acetylimidoquinone) binds to hepatocyte proteins. Liver damage and death are caused in this manner, and although such results usually take several days to occur, the antidote is ineffective unless administered during the first 10 to 12 hours. Thus, it is the unchanged acetaminophen itself which must be detected rather than any of the metabolites.
A simple and rapid procedure is therefore needed for determining acetaminophen levels in serum or other physiological fluids, which provides reproducible values and is specific for acetaminophen.
2. Description of the Prior Art
Acetaminophen, its metabolism, and its hepatotoxic properties are disclosed in Wiener, K., Annals of Clinical Biochemistry, vol. 15, p. 187 (1978); Liu, T., et al., Clin. Chem., vol. 26, p. 69 (1980); Knox, J. H., et al., Journal of Chromatography, vol. 149, p. 297 (1978); and Davis, M., et al., Journal of International Medical Research, vol. 4, p. 40 (1976). The first two of these references also disclose existing analytical techniques for measuring acetaminophen levels in serum and their inherent limitations. Homogeneous enzyme immunoassay techniques are described in Rubenstein, et al., U.S. Pat. No. 3,817,837, issued June 18, 1974 (Syva Co.).